We solved the crystal structures of the phyllosilicates shlykovite, KCaSi4O9(OH)·3H2O, and cryptophyllite, K2CaSi4O10 ·5H2O, two minerals recently discovered in the Khibiny alkaline complex (Kola Peninsula, Russia). Both minerals are monoclinic with close values of the a, b and β parameters but significantly different c parameters: shlykovite a = 6.4897(4), b = 6.9969(5), c = 26.714(2) Å, β = 94.597(8)°, V = 1209.12(15) Å3, sp. gr. P21/c, Z = 4; cryptophyllite a = 6.4934(14), b = 6.9919(5), c = 32.087(3) Å, β = 94.680(12)°, V = 1451.9(4) Å3, sp. gr. P21/n, Z = 4. The structures were solved from synchrotron diffraction data collected on the same intergrown crystal and refined anisotropically to R(F) = 0.0960 for 1147 unique reflections with I > 2σ(I) (shlykovite) and R(F) = 0.0856 for 1667 unique reflections with I > 2σ(I) (cryptophyllite). Shlykovite and cryptophyllite are representatives of two new, closely related structure types. The main structural units of both minerals are TOT blocks consisting of tetrahedral Si layers (T) and an octahedral component (O), sandwiched between them. Each T-layer consists of 4- and 8-membered rings of SiO4-tetrahedra and can be considered as a half of a double layer described in the structures of the members of the rhodesite mero-plesiotype series. A topologically closely related Si layer also forming T-fragments of TOT blocks was recently discovered in mountainite, KNa2Ca2[Si8O19(OH)]·6H2O. The O-fragment of the TOT blocks in shlykovite and cryptophyllite is formed by columns of edge-sharing Ca-centred octahedra CaO5(H2O). In both minerals K cations are located in the voids of the Si layer. The content of the interlayer space is different in each mineral: only H2O molecules in shlykovite, K atoms and H2O molecules in cryptophyllite. Mountainite, shlykovite and cryptophyllite are closely related in configuration of the TOT blocks, their main structure unit, and could be used to define the mountainite structural family.

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